Loads connected to electrical circuits can be damaged if the loads draw more current than they are configured to tolerate. For example, inductive motor loads that draw too much current from a power source can suffer from overheating, which degrades the motor life and can potentially break down the insulation in the motor windings. Additionally three-phase inductive motor loads configured to receive power from a three-phase AC power source can be damaged if the power source suffers from a phase loss or phase imbalance. A phase loss occurs when one phase of the current delivered by a polyphase power source is disconnected. A phase loss fault can occur due to a blown fuse or a discontinuous power wire in series with the conductor carrying the phase. A phase imbalance fault occurs when one or more of the phases of the current drop below some characteristic fraction of their nominal values. A phase loss fault and a phase imbalance fault can degrade the motor load by causing the motor windings to draw current unevenly and heat unevenly, which is inefficient and degrades the life of the motor and potentially overheats the insulation of the motor windings.
An electronic protection device can protect a load from fault events by monitoring the current drawn by the load and disconnecting the load from its power source upon detecting a fault event. The electronic protection device can also be self-powered, that is, the device can be powered from the same current that the device monitors. Self-powered electronic protection devices are conventionally powered from a rectified secondary winding of a current transformer (CT), with the monitored current sent through a primary winding of the CT. Conventional electronic protection devices monitor the current drawn by the load by measuring current returning to an anode of the rectifier connected to the CT secondary. This approach for measuring the current requires an inverting amplifier or a difference amplifier to condition the measurement signal for analysis within the device.
However, an inverting amplifier or difference amplifier incurs additional cost and current consumption. As a result of current consumption and cost constraints, many conventional electronic protection devices incorporate only one inverting amplifier and provide a measurement signal representative of the sum of rectified currents from all phases of a polyphase power source. Such a conventional configuration cannot measure each phase current directly without incurring additional cost and current consumption penalties by adding additional inverting amplifiers to condition the additional phases. Detection of phase loss or phase imbalance faults is improved by measuring each phase current directly for analysis within the electronic protection device. So, conventional electronic protection devices are forced to compromise between cost and current consumption on one hand, and sensitivity and performance of the device on the other.